Some critical data is missing from this "report".
Salaries stated for Government workers do NOT include their retirement, medical, etc. costs. What a contractor charges does include those costs for their people. Including those costs will make the comparison more equitable, if not tip it the other way.
Doing business with the Federal Government is expensive for a company just because of the amount of paperwork and tracking that is required. Those costs are not part of the internal Federal IT costs.
A very small percentage of federal contractors work in Government facilities. Those that do get charged for at a considerably lower rate than those that work in contractor facilities. All of the facility, training, retirement, etc. costs cited are overhead - added as a percentage of the direct contract costs. Federal contractors go through incredible machinations to cut those overhead costs. They are a major component of a competitive bid, and you can easily lose a bid if they are higher than your competitor.
Government IT can often be cheaper than contractor, but not always, and Government personnel regulations and processes can make hiring new IT personnel a year-long (or more) nightmare. Personnel limits imposed by Congress, whether designed to cut Federal numbers or to benefit contractors, can also make an agency or department go outside for IT support.
The full reasons and costs are NOT easy to tease out, and a simple analysis does NOT get you the real reasons/problems, nor does it provide for valid fixes.